(1) Field of the Invention:
The present invention relates to metal-ceramic joined, composite bodies in which a ceramic member is integrally joined to a metallic member.
(2) Related Art Statement:
Since ceramic materials such as zirconia, silicon nitride, and silicon carbide have excellent mechanical strength, heat resistance, and wear resistance, they have been put into practical use as high temperature structural materials or wear-resistant materials for gas turbine engine parts, internal combustion engine parts, etc. However, since ceramics are generally hard and brittle, they are inferior to metallic materials with respect to shapability and processability. Further, since ceramics have poor toughness, they have low resistance against impact forces. For this reason, it is difficult to form mechanical parts such as engine parts from ceramic materials only. In many cases, such ceramic materials are generally used in the form of a composite structure in which a metallic member is joined to a ceramic member.
When a metallic member is to be joined to a ceramic member, a joined body is generally obtained by shrinkage fitting a projection of the ceramic member into a recess of the metallic member or by brazing them. In this case, stress is likely to be concentrated at an end of a joined portion between the outer surface of the projection of the ceramic member and the inner surface of the recess of the metallic member. Consequently, the strength of the joined structure against bending or twisting is decreased, and the ceramic member can be broken.
In the case of the metal-ceramic joined composite body in which a projection formed at the ceramic member is integrally joined to the recess of the metallic member, in order to prevent stress concentration at the end of the joined composite body, Japanese Patent Application Laid-open No. 59-159,408 discloses a joined structure in which a groove is formed in the outer peripheral portion of the projection. Joining is effected by shrinkage fitting as a mechanical joining way, such that an end of the groove is located at an end of a joined portion between the projection of the ceramic member and the recess of the metallic member.
However, even when this structure is attained by the above-mentioned mechanical joining, an edge of the groove, provided over the entire periphery of the projection of the ceramic member, contacts the recess of the metallic member, and excessive stress concentration occurs there. Consequently, the bending strength and twisting strength of the joined composite body are decreased, and the ceramic member is likely to be broken.
Even if the edge of the groove is designed as an angular edge or a rounded edge, stress concentration may still occur there. For this reason, bending strength and twisting strength of the joined composite body also decrease, and the ceramic member is likely to be broken. Further, when the joined composite body is used in an atmosphere such as a combustion gas, the end of the joined portion is exposed to the combustion gas, so that durability of the joined composite body decreases.
Further, there have been known metal-ceramic composite bodies in which an outer surface of a projection of a ceramic member is joined to an inner surface of a recess of a metallic member by brazing, and the projection is firmly fixed, in the thus joined portion, to a brazing metal present between the outer surface of the projection of the ceramic member and the inner surface of the recess of the metallic member through chemical joining over the entire surface of the projection.
Consideration is now made of a case where a projection of a ceramic member is to be joined to a recess of a metallic member by brazing. When temperature is lowered from a solidifying point of the brazing metal to room temperature, shrinkage amounts of the metallic member or the brazing metal are generally greater, since there is a difference in the coefficient of thermal expansion, i.e., since the metallic member or the brazing metal has a greater coefficient of thermal expansion while that of the ceramic member is smaller. However, as mentioned above, in the structure in which the projection of the ceramic member is firmly fixed to the brazing metal over the entire contact surfaces between them through chemical joining, the brazing metal is firmly fixed to the ceramic member due to shrinkage of the metallic member or the brazing metal during cooling. Consequently, the brazing metal cannot slip, relative to the ceramic member at their joining interface so that shrinking forces of the metallic member or the brazing metal act upon the ceramic member and excessive tensile stress occurs in the ceramic member. Further, no sufficient investigations have been made upon the relationship between the surface of the tip end of the projection and the bottom surface of the recess. The relationship between the diameter and the length of the joined portionship has not fully been considered.
Therefore, tensile stress greatly concentrates particularly upon the end of the joined portion of the ceramic member, which reduces resistance of the joined composite body against bending or twisting, and deteriorates reliability.